Sorry, I have upgraded tons of iphones to 4.0.1 and have never seen that issue where wifi wont work on one firmware while still working on the previous firmware. Sounds like it might be an Apple problem to me.

The baseband on 4.0.1 is still unlockable by ultrasn0w. The update that is supposed to come out next week (4.1) has a baseband update that kills the ultrasn0w unlock so you will want to stay away from that update until a new unlock comes out. You will need either a original carrier sim or phonebook sim to activate using this method, so if you do not have one this method will not work for you.
However there is a chance if you have an old bootrom 3GS that you can create a custom 4.0 firmware and restore to that. Download idetector from here iH8sn0w.com | Jailbreak your iPod touches and iPhones
You will have to put your iphone into dfu mode for idetector to tell you if you have old bootrom or not. To get to dfu mode plug in iphone, then hold home and power button for exactly 10 seconds, then release power button while continuing to hold home button for an additional 10 seconds. Let me know what idetector says and we can go from there.

hi i have an iphone4.It is 4.0 firmware.I had downloaded a file from cydia and since then no application is working on iphone.I installed tiny umbrella but it has an shsh for firmware 4.02.It does not save my ssh for 4.01 or 4.0.
I tried evrything by changing my host file but the itune always give an error of 3xxx
Is there any way i can restore my my iphone4 as i can only recieve calls and do nothing on it.Please help